From an aircraft standpoint, the speeds shown in many recent trajectory hypotheses seem unreasonably low.
Aircraft tend to attain maximum cruise range over a rather small range of Mach numbers, typically not more than a few percent variation. Boeing provides a paltry amount of hard numbers here:
http://www.boeing.com/assets/pdf/com...f/777_perf.pdf
The 200-ER is on pgs. 4-6, with Rolls Royce engines on pg. 6. Note that they show data for only 2 engine types (Trent 884 and Trent 895), whereas 9M-MRO had Trent 892 engines (
ASN Aircraft accident Boeing 777-2H6ER 9M-MRO Indian Ocean ). But in either case, Boeing's recommended cruise speed is 0.84 M.
I haven't found the difference between the 895 and 892 engines, other than this brief press release:
ROLLS-ROYCE TO OFFER MORE POWERFUL TRENT ENGINE -- August 05,1998 /PR Newswire UK/
"Rolls-Royce plc announced today that it is to offer a more powerful derivative of the successful Trent engine family for the Boeing 777. The 95,000lb thrust Trent 895, which will be to the same production build as today's Trent 892, will be certificated in 1999, ready for entry into service in 2000."
This Boeing magazine article discusses cruise speed selection generically:
http://www.boeing.com/commercial/aer...7_article5.pdf
Fig. 1 is purely notional, i.e., not for a particular aircraft. However, the implication is that maximum range cruise occurs over a fairly small range of Mach numbers.
I can recall a flight manual from 40 years ago that had many graphs of specific range (nautical miles per pound of fuel), with the curves looking rather like parabolas, with Mach number on the horizontal axis, and specific range on the vertical axis. In other words, much like the notional Figure 1 in the Boeing magazine article.
Finding a similar graph on the web is another matter. Probably out there somewhere, but I haven't found it yet. Possibly in this file, or one of the recommendations at the right side of the page (I'm out of Scribd credits at the moment):
http://www.scribd.com/doc/55989739/B...raining-Manual
Here's a discussion from 2001 on best cruise speed of a B777-200ER with Trent 892:
B777-200ER Economical Cruise Speed — Tech Ops Forum | Airliners.net
Roughly 0.82 to 0.85 Mach. A speed of 0.84 M is about 480 kts (assuming standard atmosphere, at or above tropopause). The range from 0.82 M to 0.85 M is roughly 470 to 490 kts. True airspeed, not groundspeed. The range of Mach numbers is likely due to winds input to the flight management computer, i.e., slow down with a tailwind, speed up with a headwind.
This simulator site shows a range chart on pg. 47 (pg. 11 indicates Trent 892 engines);
http://www.deltava.org/library/B777%20Manual.pdf
The chart is a bit hard to read, but the multiple curves account for various no fuel weights and fuel loads. It likely also includes takeoff, climb, descent, and landing, i.e., not purely cruise at altitude.
The Boeing performance table on pg. 6 of the first link above lists the 777-200ER operating empty weight as about 142,000 kg.
Reportedly, the aircraft had about 49,000 kg of fuel at departure (
https://twitter.com/jonostrower/stat...12316941299712 ).
It also carried 227 passengers and 12 crew members (
Investigation: AE-2014-054 - Technical assistance to the Department of Civil Aviation Malaysia in support of missing Malaysia Airlines flight MH370 on 7 March 2014 UTC ). Say 240 people total, and assume 100 kg each, including baggage, or 24,000 kg.
Then the no fuel weight would be about 166,000 kg, and the gross weight at takeoff would be about 215,000 kg. Probably a few thousand kg more for cargo.
The chart on pg. 47 of the simulator ("Virtual Delta") PDF has a gross weight of 217,700 kg in the 7th curve from the left. Follow that curve left and up to 166 (thousand kg) on the left hand scale, and you get on the order of 3,500 nm range.
Dividing that range by 7.6 hrs of flight time gives an average speed of about 460 kts, or about 0.80 M. Takeoff and climb are slower, so that's not inconsistent with cruise at 0.84 M.
However, we have no knowledge that the aircraft flew 3,500 nm. Suppose for example that it flew slower to attain max endurance rather than max range. But that would imply that it would have sufficient fuel to fly longer, perhaps as much as an hour.
There is no requirement for a max range or max endurance trajectory, and there are many possible explanations for a speed well below that. But it doesn't seem like the most likely scenario.
A few academic sources for aircraft performance:
40 year old RAF Cranfield document, "Range Performance In Cruising Flight", lists equations and criteria for max range cruise:
Range Performance in Cruising Flight
Charts from Arizona State University:
http://enpub.fulton.asu.edu/aero/mae...rmance%20I.pdf
Notes from Virginia Tech:
http://www.dept.aoe.vt.edu/~lutze/AO...&endurance.pdf
MIT lecture:
http://web.mit.edu/16.unified/www/FA...etNoteseps.pdf
From the RAF Cranfield paper, Peckham gives an example for which the maximum range speed is about 20% higher than the maximum endurance speed. He neither states nor implies that this is a general result. If it were, one might infer that the 777-200ER cruise speed of 0.84 M would imply a max endurance speed around 0.70 M. That's still a good deal higher than the speeds in the recent best estimates of trajectories.